You don't have to win the lottery or even earn a huge paycheck to retire early, Justin tells CNBC: "It really just comes down to saving some of your income, setting it aside and letting it grow."

The key to saving more than half your income, he says, is to keep the "big three expenses" — housing, transportation and food — as low as possible: "Look at those top expenses and see if there's any negotiating room."

Source: The McCurry family

The McCurry family

By cutting back on "the big three," the McCurry's can keep their annual expenses incredibly low for a family of five: at about $40,000.

They stayed in the starter home they bought out of grad school and paid off their mortgage in 2015, meaning they only cover utilities and maintenance. In terms of transportation, "we kept the cars that we bought brand-new in college for 16 years and just replaced them last year," says Justin.

As for food, they budget $500 a month for groceries. You can read more about how they feed a family of five for $500 a month on their blog, Root of Good.

"If you look at my spending compared to most people, the two main areas I save the most on are rent and car payments," says "The Money Wizard," who keeps his fixed costs at or below $1,000 a month. "I drive a 13-year-old truck that's completely paid off and I split rent with my girlfriend in Minneapolis."

His perspective is similar to that of the McCurry's. You don't need inheritances or windfalls to retire early, he writes: "Just lots of saving, basic investment, and a desk job in the finance industry that places my salary barely above the median household income."